The following are some resources you can use to explore a topic in depth, watch tutorials, and see examples:
Khan Academy - An extensive database of tutorial videos, notes, example problems, self-quizzes. You can even create an account and track your progress across different math strands.
Wolfram Alpha - An amazing resource. Just go look at it. Search anything, and it will tell you ... everything.
Math Open Reference - A collection of simple Geometry and Calculus applets. Very helpful for visualizing dynamic concepts.
Paul's Online Math Notes - College-level (Calculus I-III, Ordinary Differential Equations) math lecture notes from a University Professor. Bare-bones, but very clear and good for brushing up on techniques.
Patrick JMT - This was shared with me by a parent. Thank you! It looks like a great and extensive database of high-quality tutorials.
YouCubed
Stanford Professor Jo Boaler, who has collaborated with mindset and grit researchers Carol Dweck and Angela Ducksworth, has a wonderful website about math education for students, parents, and teachers. She presents research about learning and education as well as tools for learning. There is even a short online course available to students and parents who'd like answers to the question, "How to Learn Math".
Information from Professor Darryl Yong at Harvey Mudd: "Harvey Mudd College offers free tutoring for math and science over the phone. Yes, FREE!!" Details: Sundays - Thursdays 6-9pm Number: 1-877-8-ASK-HMC Grades 4-12 welcome
The homework hotline is staffed by Mudd students who are trained to offer help without just giving answers to kids. They do a great job.